Field of the Invention
The invention relates to a thermal protection circuit having a first bipolar transistor, whose emitter connection is connected to a terminal for a reference potential, whose collector connection is connected to a first current source and whose base connection is connected to a tap of a voltage divider. One terminal of the voltage divider is connected to the terminal for the reference potential.
The task of such thermal protection circuits, which are used in integrated power circuits for example, is to switch off circuit components having a high dissipation power when a defined temperature threshold is exceeded. This is done to protect the entire circuit, usually an IC, from being destroyed when it is not cooled. This necessitates a temperature-dependent switching signal which, at temperatures above the defined temperature threshold, has a value which can be distinguished unambiguously from values of the switching signal at temperatures below the defined temperature threshold. In the case of already known thermal protection circuits of this type, the pronounced dependence of the collector current on the temperature in the case of bipolar transistors connected in a common-emitter connection is utilized for generating the switching signal. Given a predetermined collector current, the base-emitter voltage of a bipolar transistor operated in a common-emitter connection decreases by a specific value per kelvin of a temperature increase. This specific value is approximately 2 millivolts per kelvin of the temperature increase in the case of silicon-based bipolar transistors. Since the collector current is, in turn, exponentially dependent on the base-emitter voltage if the transistor is in the linear control range, the collector current is thus exponentially dependent on the temperature, with the result that the collector current rises exponentially for a predetermined base-emitter voltage and temperature increase. If the current supplied by the current source connected to the collector connection no longer suffices to keep the bipolar transistor in the linear control range in the event of rising temperatures, the transistor reaches saturation and the collector potential decreases rapidly in relation to values at lower temperatures, whereby an unambiguously distinguishable switching signal is produced. In the case of already known thermal protection circuits which utilize such temperature dependencies of common-emitter bipolar transistors, a reference voltage source which is as exact as possible and is independent of temperature is required in order to set the base-emitter voltage and hence the transistor operating point. In order to generate such a reference voltage, it is possible to use for example bandgap circuits, as is described in "Smart-Power ICs" by Botti/Stefani, Springer Publishers, 1996, page 424 ff. or in Published European Patent Application EP 0 618 658 A1 assigned to the company SGS Thomson.
Circuits of this type have the disadvantage that the temperature accuracy of the protection circuit is greatly dependent on the accuracy of the reference voltage source. Another disadvantage is the considerable outlay on circuitry.
U.S. Pat. No. 5,589,792 describes a thermal protection circuit having two bipolar transistors of the same type, the emitters thereof being connected to a common terminal connection. Furthermore, two current sources are provided, which in each case impress a predetermined current into the collector connections of the two transistors. A third transistor enables a current flow through the first transistor, this current flow being caused by the first current source. A voltage divider controls the base of the second transistor in such a way that part of the base-emitter voltage of the first transistor is present. A signal can then be tapped on the collector of the second transistor when a predetermined temperature is exceeded. Thus, it is possible to dispense with a complicated circuit configuration for generating a reference voltage.